Early-Stage Evolution of the Neo-Y Chromosome in Drosophila albomicans

Chia-Hao Cheng, Ching-Ho Chang, and Hwei-yu Chang (2011) Early-stage evolution of the neo-Y chromosome in Drosophila albomicans. Zoological Studies 50(3): 338-349. Numerous theories have specified that an originally autosomal neo-Y chromosome arm is expected to undergo degenerative evolution. Neo-sex chromosomes of Drosophila albomicans originated from 2 Robertsonian translocation events, one for X and the other for Y, between ancestral Drosophila sex chromosomes and a pair of autosomes homologous to the 3rd chromosomes of its sibling species D. nasuta. Since the neo-sex chromosome in D. albomicans is still evolutionarily young, we used genetic approaches to reveal changes in the entire neo-Y chromosome. Non-disjunction is an indicator used to investigate differences between homologous chromosomes. In this study, we first confirmed that no male recombination had occurred in hybrid males of these 2 sibling species. With the aid of molecular marker genotyping and direct karyotyping of aneuploid offspring produced through specially designed crosses and backcrosses of fertile hybrids, we found that the non-disjunction rate was significantly higher in hybrid males with the neo-Y chromosome than in hybrids without it. The high nondisjunction rate made it possible to generate 3,X,X/neo-Y F-2 females and X,neo-Y/neo-Y F-3 male offspring which can reveal recessive effects of the homozygous 3rd chromosome arm. Results of this aneuploid study revealed severe recessive inviability of the neo-Y chromosome. Our results further suggested that increased non-disjunction in hybrid males with the neo-Y chromosome is likely due to changes that occurred on the Y arm, whereas recessive deleterious alleles might be located on the 3rd arm of the neo-Y chromosome. Taken together, the elevated non-disjunction rate and severe recessive inviability revealed significant changes in the neo-Y chromosome at this early stage of chromosome evolution in D. albomicans. http://zoolstud.sinica.edu.tw/Journals/50.3/338.pdf